Descriptions

Nitrogen (N), phosphorus (P), and sulfur (S) fertilization
trials with Stephens winter wheat (Triticum aestivum L.) were
conducted on the Columbia Plateau under minimum and no tillage
conditions. The crop rotation was either wheat-summer fallow
(summer fallow) or wheat-wheat (recrop).
Nitrogen variables included: N rates, N timing, and N
source treatments. The N fertilizer rate required to maximize
yield depended on the amount of precipitation received, soil
nitrate levels, and yield potential. Increasing precipitation
increased yield potential (within limits partly determined by the
available water holding capacity of the soil) which increased the
N requirement for maximum grain yields. Soil nitrate varied in
its ability to reduce the requirement for N fertilizer. At low
soil nitrate levels, an increase in soil nitrate caused a corresponding decrease in N fertilizer requirement; however, at moderate to high soil nitrate levels, N fertilizer requirements
were reduced less than the increase in soil nitrate. Nitrogen
timing treatments consisted of N applied in the spring of summer
fallow (SS), at seeding (SD), and in the spring of the crop year
(SC). In a wet year (1983-84), SC and SD applied N gave similar
yield results. In a dry year (1984-85), SS and SD applied N gave
similar yield results. Overall, N applied at SD gave the highest
yields. Urea, ammonium chloride, ammonium sulfate, and urea
phosphate were compared as N sources. Urea phosphate tended to
increase yield more than urea plus phosphorus. Ammonium chloride
tended to lower protein content and increase 300-kernel weight
more than urea and ammonium sulfate, possibly because of chloride
effects on plant water potentials.
Responses to phosphorus fertilizer were not accurately
predicted by soil P test levels. Cold soil temperatures at
planting (due to high elevation and/or late planting dates) in
addition to high yield potentials, seemed to correspond to P
responsive sites better than soil P test levels.
At experimental sites where S soil test levels were above 2
ppm in the top 30 cm, wheat did not respond to S fertilizer.
When S soil test levels were below 2 ppm in the top 30 cm, a
response to S fertilizer was recorded approximately 17%, of the
time.